During microelectronics manufacturing, a semiconductor wafer is processed through a series of tools that perform lithographic processing, followed by etch processing, to form features and devices in the substrate of the wafer. Such processing has a broad range of industrial applications, including the manufacture of semiconductors, flat-panel displays, micromachines, and disk heads.
The lithographic process allows for a mask or reticle pattern to be transferred via spatially modulated light (the aerial image) to a photoresist (hereinafter, also referred to interchangeably as resist) film on a substrate. Those segments of the absorbed aerial image, whose energy (so-called actinic energy) exceeds a threshold energy of chemical bonds in the photoactive component (PAC) of the photoresist material, create a latent image in the resist. In some resist systems the latent image is formed directly by the PAC; in others (so-called acid catalyzed photoresists), the photo-chemical interaction first generates acids which react with other photoresist components during a post-exposure bake to form the latent image. In either case, the latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent etch processing, the resulting resist film pattern is used to transfer the patterned openings in the resist to form an etched pattern in the underlying substrate. It is crucial to be able to monitor the fidelity of the patterns formed by both the photolithographic process and etch process, and then to control or adjust those processes to correct any deficiencies. Thus, the manufacturing process includes the use of a variety of metrology tools to measure and monitor the characteristics of the patterns formed on the wafer. The information gathered by these metrology tools may be used to adjust both lithographic and etch processing conditions to ensure that production specifications are met.
Referring to FIG. 1, a typical lithographic and etch production manufacturing line 10 for manufacturing semiconductors is illustrated schematically. One or more semiconductor wafers 5 are processed in the manufacturing line 10 along the direction 100. A photocluster 110 contains photolithography tools, including track tools 111 for depositing and baking resist on the wafer, imaging a pattern on the wafer plane (e.g. exposure tools 112), and post-exposure track tools 113 for baking and developing the exposed pattern on the resist film. After photolithography, various tools are used to measure characteristics of the patterns formed on the resist. For example, an overlay measurement tool (OLM) 120 is used to ensure that the patterns formed on the resist layer are sufficiently aligned to previously formed patterns on the wafer. A scanning electron microscope (SEM) 130 is typically used to measure the width of critical dimensions (CD) of pattern features. The measurements from the metrology tools 120, 130 may be communicated to the photocluster 110 and the etchcluster 140 (typically including an etch chamber 141), as indicated by the dataflow path 135, to allow adjustments to the process conditions in accordance with those measurements.
These measurements are assessed in a disposition step 125, where a decision must be made as to whether the wafer 5 must undergo a rework process 101 in which the resist is stripped from the wafer 5 and sent back to the photocluster 110 to reapply the resist pattern under modified lithographic conditions. If the resist pattern meets production specifications, the wafer 5 can continue to be processed by the etchcluster 140. These decisions are typically made based on a limited number of measurements for each wafer; for example, about 2-3 overlay measurements at about 20 sites and only 1 CD measurement at 5-10 sites per wafer. This limited number of measurements is required to maintain a reasonable throughput processing rate of about 30 seconds per wafer or about 100 wafers per hour.
If the wafer 5 meets overlay and CD measurement requirements, the wafer 5 proceeds to processing in the etchcluster 140 where the resist pattern is transferred to the wafer substrate. Once again, the resulting pattern on the substrate will undergo measurement by metrology tools such as an in-line SEM 130 or atomic force microscope (AFM) 150. Post-etch metrology data from metrology tools 130, 150 may be fed back to other tools in the line along data flow paths 135 so that adjustments to process conditions may be made.
Periodically, more extensive off-line measurements 15 may be made using tools similar to those used in-line, such as an OLM 120, an SEM 130 and an AFM 150, and may also include other tools such as a film thickness measurement tool (FTM) 160 and an electrical probe measurement tool (EPM) 170.
It would be desirable to obtain even more measurements at more sites and on all wafers. Thus, referring to FIG. 2, a more desirable hypothetical wafer processing system 20 may include tools such as an FTM 160 and an OLM 120 within the photocluster 110. Other metrology tools and methods may also be beneficial, such as scatterometry metrology (SCM) 180 and microscopy (MCR) 185, which would provide information that is not typically provided today. Although such a hypothetical processing system 20 would have increased metrology capability over conventional systems, this increased capability would come at the expense of increased complexity and cost.
In recent years, so-called “scatterometry” techniques have been developed that enable optical metrology of periodic structures without the need for sophisticated hardware such as an SEM or AFM. The principle of scatterometry is that detailed information about small patterns can be extracted from the reflected or zero-order diffracted energy of grating-like patterns. Conventional SCM uses reflected energy from patterns on the wafer and compares the signal of the reflected energy to determine pattern characteristics. SCM has the advantage of relative speed and simplicity, but requires the development of extensive libraries of signals to which the reflected signal can be matched. Those libraries are costly and time-consuming to develop, and also require computer servers 190 and associated databases to perform the required comparisons. Scatterometry may be added as well as to off-line metrology systems 25 to improve the quality and quantity of information and subsequent control of lithographic and etch processes. For example, Littau et al. (U.S. Pat. No. 6,429,930) has described using scatterometry to determine the center of focus by measuring a diffraction signature and comparing it to a library of diffraction signatures at different incident angles, wavelengths and/or phases to determine the center of focus. However, scatterometry is computationally intensive, and requires server farms and databases containing signal libraries, thus increasing complexity and cost. Scatterometry requires the simultaneous determination of multiple free parameters pertaining to both the film stack and the target pattern. Its success depends on detailed a priori knowledge of the film stack and pattern characteristics that are often indeterminate. Since conventional scatterometry is not a differential measurement, its application to CD measurement is susceptible to noise: for example, measurement variation such as illumination, wavelength, detector response, target alignment; process variation such as film thicknesses and optical properties. Conventional scatterometry is also restricted to detection of the zeroth diffracted order, which can be used to characterize film thickness, but has typically poor signal to noise ratio in distinguishing signal signatures due to the target CD from those due to the film stack. The targets used with scatterometry must be large enough so that the illumination is contained within the target (i.e. the illumination must be fully landed within the target area), which takes up more chip area than typical CD or overlay targets. In addition, scatterometry capability degrades as target features become more isolated (the ratio of target CD to the target period decreases). Since CD sensitivity to defocus increases with the degree of isolation, the ability to measure defocus, a critical lithographic processing parameter, requires the measurement of isolated features.
It is desirable to control the photolithographic process conditions (e.g. exposure dose and defocus) to ensure the highest quality image. The principal determinant of the photoresist image is the surface on which the exposure energy equals the photoresist threshold energy in the resist film. “Exposure” and “focus” are the variables that control the shape of this surface. Exposure, set by the illumination time and intensity, determines the average energy of the aerial image per unit area. Local variations in exposure can be caused by variations in substrate reflectivity and topography. Focus, set by the position of the photoresist film relative to the focal plane of the imaging system, determines the decrease in modulation relative to the in-focus image. Local variations in focus can be caused by variations in substrate film thickness and topography.
The use of microscopy (MCR) 185 can be used in conjunction with specially designed metrology targets to monitor dose and focus, as described further below. The lithographic patterning of wafers in semiconductor manufacturing depends on controlling the lithographic process to guarantee that the various pattern features stay within a common process window. This process window is the parameter space over which all pattern tolerances are met. Thus, accurate measurement and control is required of two fundamental parameters of lithography processing, specifically dose and focus (or defocus). Dose specifies the mean energy of the image, and defocus is the lowest order aberration that causes image degradation. Lithography control must be based on the predetermined response of measurable pattern attributes to dose and defocus. It would be desirable to control dose and focus in-line during the manufacturing process.
One method of characterizing the response of patterns to dose and defocus is through the use of a focus exposure matrix (FEM). A grid or matrix of test patterns is formed in which the grid elements are processed through a range of focus and dose settings, and pattern attributes within each grid element are measured to characterize the lithographic process.
The measurement of pattern attributes is typically performed using either a scanning electron microscope (SEM) or an optical tool to form images of the patterned wafers (e.g. an FEM wafer). However, SEM metrology is expensive to implement, is relatively slow in operation and is difficult to automate.
Methods for using microscopy for obtaining dose and focus have been described by Ausschnitt et al. (e.g. C. P. Ausschnitt, “Distinguishing dose from defocus for in-line lithography control,” SPIE, Vol. 3677, pp. 140-147 (1999); Ausschnitt et al., U.S. Pat. No. 5,965,309; Ausschnitt et al., U.S. Pat. No. 5,976,740). Ausschnitt et al. have disclosed dual-tone metrology targets (referred to as “schnitzls”) for characterizing dose and focus. The “tone” of a lithographic pattern is determined by the presence or absence of resist material which is normally deposited in a layer or film on the surface of the substrate of a wafer to be etched. Patterns are either resist shapes on a clear background or the absence of resist shapes (i.e., spaces) in a background of resist material. Complementary tone patterns can be formed by interchanging the areas that are exposed during the lithographic process. These tone patterns may be created in resist material by preparing masks with opaque and transparent areas corresponding to the shapes or spaces to be created on the resist material, and then using a source of radiation on one side of the mask to illuminate and project the mask shapes and spaces on to the resist layer at the opposite side of the mask. The dual-tone metrology targets disclosed by Ausschnitt et al. have differential responses (for example, by taking advantage of differential bias and line shortening effects) to dose and focus that can be measured using microscopy systems. A further advantage is that the same microscopy system can be used to measure overlay as well as dose and focus. However, the roughly symmetric sensitivity of schnitzlometry to lithographic focus deviation leads to ambiguity regarding the sign of the focus deviation. In addition, this “schnitzlometry” method requires high quality microscopy and focusing capability in which a precise image of the schnitzelometry and overlay targets must be captured in order to obtain the required measurements. Precise, in-focus image capture adds to the time required for measurement and makes the measurement susceptible to process and environmental variations that might exist within photo- and etch-clusters.
Conventional overlay metrology also depends on microscopy and is susceptible to similar lens quality, focusing and process variation issues. In particular, the use of microscopy introduces error sources such as tool-induced shift (TIS), errors due to tool calibration and optical alignment variation, and wafer-induced shift (WIS), errors due to process nonuniformities in both underlying layers and the overlay target itself.
Accordingly, there is still a need for an inexpensive, rapid, in-line method and system of measuring and controlling the patterning of lithographic and etch processes; one that is primarily sensitive to the pattern dimensions, both on a single level and relative to a prior pattern level, and insensitive to the properties of the film or films in which the patterns are formed and the underlying film stack and substrate.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an integrated metrology system, including an in-line measurement and control tool, test patterns and evaluation methods for determining lithographic and etch process conditions as well as overlay error whereby one pattern group is capable of distinguishing between exposure, focus and etch problems and a second pattern group is capable of measuring two-dimensional overlay error in semiconductor pattern processing, and the measurement of both groups may be simultaneous.
It is yet another object of the present invention to provide a method of evaluating lithography parameters, such as focus and exposure, and etching parameters, such as rate and isotropy, which is easy and inexpensive to utilize.
It is yet another object of the present invention to provide a single apparatus capable of determining critical dimension, profile attributes (e.g., sidewall angle, thickness loss), exposure and focus conditions, overlay error, and film thickness characteristics.
It is yet another object of the present invention to provide a means of determining corrections to the lithography and etch process parameters to sustain optimum patterning performance.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.